QUALITY CONTROL IN PERSONAL MONITORING SERVICE IN TVO NPP, FINLAND

Transcription

1 QUALITY CONTROL IN PERSONAL MONITORING SERVICE IN TVO NPP, FINLAND MSc Antti Matilainen Doseco Ltd P.O.Box 229, Jyväskylä Finland GENERAL RADOS TLD System is used for personal monitoring in TVO, Olkiluoto. System includes: two Computer controlled Dosacus RE-1 TLD Readers, Reference Irradiator, 90 Sr Oven, Memmert, Tv30u Rados Explorer WinTLD version , TLD Server version dosimeter card, 4 TLD chips o 2 pieces of 7 LiF:Mg, Ti (called LiF, commercial name TLD-700) o 2 pieces of Li 2 B 4 O 7 :Mn, Si (called LiB) plastic cover model TVO measured quantity: H P (10) and H P (0,07) monitoring is made by monthly basics. Personal monitoring is operated by Doseco Ltd according to ISO/IEC 17025:2005 1) and IEC 1066 standards 2). QUALITY CONTROL Annual calibration, cleaning Annual calibration is based on irradiations by the secondary standard laboratory (STUK, in Finland). Irradiation qualities are 60 Co, 137 Cs and ISON300. From these irradiations basic calibration factor Krem, unit msv/ one pull of 90 Sr irradiator, is calculated. The dosimeter system homogeneity and linearity are checked once in a year according to the standard IEC The readers and the irradiator are cleaned up by manufacturer s authorised personnel yearly. Annealing Dosimeters are annealed in the reader. Preheat time is 1,5 s and measuring time 10,5 s. Temperature is 316 C. Annealing is checked by HEHKU 1.5.T software (Fig. 1). From the report shown in Fig. 2 one can see if the dosimeter is annealed properly. Operator can set parameters to be checked (see Fig. 2). The maximum measured value and the difference between LiF and LiB chips are checked by HEHKU-software. If the value (maximum or difference) is out of the limit the dosimeters are annealed again.

2 Kiteet 1&4 = Chips 1 &4 Dosimeter Time Reannealing Kyllä= Yes Used parameters for glow curve analysis, see Fig. 5 Figure 1. HEHKU-sofware. Max measured value Min measured value, if below this then no checking Kide = chip Difference between chips 1&4 and 2&3, if more than 300 % or 2000 measured units then reannealing Figure 2. Parameters in annealing. Reader calibration Before reading of the used dosimeters, reader sensitivity is checked by 90 Sr irradiator by using 40 TLD chips. Measured counts, reference light values and dark current values (Fig. 3) are tabulated. The limit for sensitivity difference is ±2 %.

3 LiB LiF LiB LiF Tld -reader`s follow-up sheet limit 2% limit 2% Ref. light Ref. light Dark current Date KReader(A) KReader(B) Ref.light mean Difference Dark current mean D Kreader (A) D Kreader (B) ,7 % ,76 % 0,47 % ,3 % ,80 % 1,85 % ,6 % ,35 % -0,25 % ,5 % ,45 % -0,22 % ,2 % ,76 % -0,82 % ,5 % ,47 % 0,40 % ,2 % ,91 % -0,26 % ,0 % ,83 % 0,83 % ,3 % ,35 % 0,14 % ,1 % ,90 % 0,10 % ,1 % ,11 % 0,35 % ,0 % ,41 % -0,49 % ,1 % ,16 % -0,43 % ,5 % ,31 % 0,66 % ,5 % ,99 % -0,28 % ,0 % ,69 % 1,99 % ,5 % ,03 % -0,47 % ,2 % ,97 % -0,23 % ,3 % ,83 % -0,20 % ,3 % ,95 % 1,01 % ,3 % ,25 % -0,55 % ,0 % ,22 % 1,85 % ,2 % ,30 % -0,86 % ,1 % ,04 % -0,67 % ,2 % ,66 % 0,14 % ,1 % ,63 % 0,83 % Figure 3. Data sheet for reader quality measurements. Reader alarms The reader checks the dark current value and the reference light value before reading each pellet. In Fig. 4 are shown the used TLD Server alarm parameters 3). Figure 4. TLD Server parameters Dose reading The read out glow curves are checked by HEHKU 1.5 T software (Fig. 5). From the obtained report one can see if there are some abnormal features at glow curve.

4 Date Time Dosimeter Chip Comment for checking: Ero = difference, ROI = region of interest -> too high value To service x = yes Recalculate x = yes, 5966 = new value List of used parameters Used parameters Figure 5. Glow curve analysis Operator can choose quality checks. In our system two LiF chips for H p (10) dose are used and the software calculates the difference between these two LiF chips. Operator can set two region of interest (ROI1 and ROI3). Channels for ROI3 (beginning of glow curve), channels (LiB) and (LiF) for ROI1 (end of glow curve) are used. If there are spikes or if the half wide of the glow curve is not normal the software produce a report. In a case of large dose, you can get a report by the software. If there is need for re-calculation of the dose, software gives a new value for that chip. Annual blind test Every year about 30 dosimeters are sent to STUK. Measured values are plotted in so-called trumpet 4) curve (Fig. 6). Irradiation qualities used can be seen on Fig. 6. The plotted value is a ratio of the measured H p (10) dose value and the reference dose value. When the TLD system is working properly the ratio should be 1. As can be seen from Fig. 6 the TLD system quality is quite good. Cleaning of chips TLD chips can be cleaned by washing in the ultrasonic bath (Elmasonic). New TLD chips New TLD chips go through the quality test, where background and sensitivity are measured. In the quality test +30% higher background value compared to used chips average material background is

5 acceptable. Also ± 30% difference for sensitivity compared to the used chips average sensitivity is acceptable. TVO: sokkotesti Blind , Hsyväannos P (10) Ratio between Mitatun ja measured todellisen and annoksen true dose suhde 2,50 2,00 Cs-137 1,50 1, kev 0,50 Co-60 0,00 0,10 1,00 10,00 100,00 Todellinen True dose annos /msv (msv) Figure 6. H P (10) blind test results CONCLUSIONS The quality system is very good guideline for personal monitoring service. The documented work instructions, standard oriented personnel, daily tests, annual tests, cleaning and calibrations guarantee a good quality for dose reading. REFERENCES 1) SFS-EN ISO/IEC 17025:2005. General requirements for the competence of testing and calibration laboratories. 2) International standard IEC 1066:1991. Thermoluminescence dosimetry system for personal and environmental monitoring. 3) WinTLD User`s Manual, version 2.0: Rados Technology Ltd. 4) Technical recommendations for monitoring individuals occupationally exposed to external radiation. Radiation Protection 73. EUR European Commission; 1994.